[SYCL] Don't return empty platforms (#7923)

collecting empty platforms leads to confusion for things like
`platform.has(aspect)`

Author: cperkinsintel

sycl/source/detail/platform_impl.cpp

@@ -126,10 +126,11 @@ std::vector<platform> platform_impl::get_platforms() {
           // insert PiPlatform into the Plugin
           Plugin.getPlatformId(PiPlatform);
         }
-        // The users of (deprecated) SYCL_DEVICE_ALLOWLIST expect that
-        // platforms with no devices will not be reported.
-        if (!SYCLConfig<SYCL_DEVICE_ALLOWLIST>::get() ||
-            !Platform.get_devices(info::device_type::all).empty()) {
+
+        // The SYCL spec says that a platform has one or more devices. ( SYCL
+        // 2020 4.6.2 ) If we have an empty platform, we don't report it back
+        // from platform::get_platforms().
+        if (!Platform.get_devices(info::device_type::all).empty()) {
           Platforms.push_back(Platform);
         }
       }